In testes of adult rats, Sertoli cells do not divide, and as a result comprise a non-expanding population. Thus, the number of these cells in adults is established during perinatal life when they do proliferate. However, little is known about the factors regulating this proliferation. Therefore, in the proposed research, Sertoli cells of rats will be studied during pre-and post-natal life to correlate the ontogenic pattern of Sertoli cell proliferation In Vivo and In Vitro with parameters that may be related to control of their division: 1) effect of FSH and /or cyclic AMP on proliferation, 2) appearance of FSH binding site on these cells, 3) FSH-mediated aromatization of androgen to estrogen, 4) generation of reducing equivalents required by aromatase, and 5) proportions of acid-labile and -stable DNA in their nuclei. These parameters will be studied in the intact tests In Vivo and in organ culture, as well as in cell culture, where Sertoli cells can be isolated from other testicular elements. Autoradiographic, biochemical and quantitative cytochemical methods will be used. The hypothesis that presumed control of Sertoli cell division by FSH involves aromatization of androgen to estrogen will also be explored by treating pups with a specific aromatase inhibitor. Since Sertoli cells only divide during perinatal life, prior to puberty, it is clear that any perturbation of this process in the young animal could have far-reaching consequences for fertility of the adult. Therefore, long-term studies will be aimed at establishing that the perinatal period is crucial in insuring reproductive capability of the adult male. Proliferation of Sertoli cells will be interfered with in pups by inhibiting Sertoli cell division with an anti-proliferative agent or by reducing levels of plasma FSH. Morphometry will be used to establish that these pups, as adults, have abnormally few Sertoli cells. Fertility of the perinatally-treated adults will be compared to that of normal males by observing the status of spermatogenesis in their testes, the output of androgen binding protein by their Sertoli cells, and their ability to produce offspring. The findings of this research should contribute significantly to our understanding of the importance of perinatal development in insuring a fertile adult male and should provide valuable information on possible causes underlying problems of infertility in males.